1. Field of the Invention
This invention relates to an Si3N4 sintered body having a high thermal conductivity as well as an excellent mechanical strength and further to a method for producing the same.
2. Description of the Prior Art
Heretofore, since an Si3N4 sintered body comprising Si3N4 as a main component has an excellent high-temperature property, the body has been used as materials for structural parts and mechanical parts used under high temperature condition. However, although a silicon nitride sintered body obtained by a prior producing method is excellent in mechanical strength such as toughness, it has been difficult to apply it to parts or members in which a high thermal-shock resistance is required because of being inferior in thermal conductivity as compared with aluminum nitride (AIN), silicon carbide (SiC), or beryllium oxide (BeO).
The reason why the thermal conductivity of the Si3N4 sintered body is low is that phonons as a carrier for thermal conduction are scattered by impurities such as oxygen ions which dissolve into the crystal of Si3N4 and form a solid solution. In Si3N4 powder which has heretofore been used as a raw material for an Si3N4 sintered body, since oxygen dissolves in the state of solid solution during the producing process, usually, oxygen of approximately 1.5% is contained. Oxygen existing in Si3N4 powder and oxygen in a naturally-formed oxide film composed mainly of SiO2 being produced at the powder surface are diffused and dissolve into the crystals of Si3N4 to form a solid solution in the sintering process, thereby causing phonon scattering.
In one of the methods for producing an Si3N4 sintered body, there is a reactive sintering-method using silicon powder as a raw material powder and nitriding this silicon powder. However, also in the reactive sintering-method, oxygen in the naturally-formed oxide film existing on particle surfaces of the silicon powder dissolves into the silicon particles and Si3N4 particles formed during nitriding process of silicon and forms a solid solution. Further, oxygen dissolves into the Si3N4 particles also in a subsequent sintering process and forms a solid solution. Such a solid solution unavoidably reduces the thermal conductivity of the resultant Si3N4 sintered body.
Therefore, a method for producing an Si3N4 sintered body having a high thermal conductivity has been studied. For example, in Japanese Patent Laid-Open No. 9-268069, a method has been proposed in which Si3N4 powder is used as a raw material and oxides of 3A group elements in the periodic table are added while the amount of oxygen and the amount of aluminum are controlled. In this method, the material is fired at a temperature of 1500 to 1800xc2x0 C., and further fired at a temperature of 2000xc2x0 C. or higher in nitrogen of 1.5 atm or higher for 5 to 10 hours. The resultant sintered body is further heat-treated in an non-oxidizing atmosphere at a temperature of 1000 to 1400xc2x0 C. Although it is said that this method provides an Si3N4 sintered body having a thermal conductivity at least 70 W/m.K, it can not be considered to be a method suitable for the mass production from the viewpoint of the energy cost because high temperature and high pressure are required in this method.
On the other hand, when an Si3N4 sintered body is prepared using the silicon powder as a raw material, by the reactive sintering method, nitriding and sintering reactions do not progress sufficiently when using only Si powder. For this reason, in order to achieve a highly densified and strengthened product by nitriding and sintering, in general, rare earth oxide or alkaline earth oxide are added. For example, Japanese Patent Laid-Open No. 7-109176 describes a method for producing an Si3N4 sintered body by reactive sintering at the temperature range of 1400 to 1850xc2x0 C. applicable in mass production in which silicon powder and a sintering aid consisting of Y2O3 powder and Al2O3 powder are used as raw materials. However, the thermal conductivity of the Si3N4 sintered body produced by this method is only approximately 40 W/m.K, which is not the value to be satisfied.
As described above, in order to obtain an Si3N4 sintered body having a high thermal conductivity, there has been proposed a method for achieving a high thermal conductivity and a high strength by sintering Si3N4 powder at a high temperature under a high pressure, but since the producing cost is increased due to an increase in the energy cost or the like, the method has not been practical. Moreover, even in the case where an Si3N4 sintered body is produced at a low price by a reactive sintering method, there is a difficulty in obtaining a high thermal conductivity although the Si3N4 sintered body obtained is excellent in mechanical strength.
In view of such previous circumstances, an object of the invention is to provide an Si3N4 sintered body having a high thermal conductivity together with a high strength property inherent in an Si3N4 sintered body, at a low cost, by a reactive sintering method.
In order to achieve the above-mentioned object, the invention provides a Si3n4 sintered body. In accordance with the present invention, a Si3N4 sintered body is produced by reactive sintering of silicon, the Si3N4 sintered body comprising crystal grains of Si3N4 and a grain boundary phase, wherein a compound of at least one element selected from the group consisting of Y, Yb and Sm is contained in an amount of 0.6 to 13% by weight in terms of oxide Ln2O3 thereof, an oxygen content in the crystal grains of Si3N4 is not more than 1% by weight, a ratio of oxygen and Ln in the Si3N4 sintered body is within a range of 0.1 to 0.8 in a molar ratio SiO2/Ln2O3 of the oxygen in terms of SiO2 to the oxide Ln2O3, the Si3N4 sintered body comprising a relative density in the range of 85 to 99.9%, a thermal conductivity of at least 70 W/m.K or more, and a three point bending strength of at least 600 Mpa.
Furthermore, the Si3N4 crystal grains are xcex2-type ones having an average grain size of not less than 2 xcexcm in terms of major axis, and in the grain boundary phase, preferably a compound lnaSibOcNd (wherein 2xe2x89xa6axe2x89xa64, 2xe2x89xa6bxe2x89xa63, 0xe2x89xa6cxe2x89xa67, 2xe2x89xa6dxe2x89xa64), more preferably a compound Yb4Si2O7N2 is contained.
A method for producing the Si3N4 sintered body according to the invention comprises:
mixing 80 to 99% by weight of a silicon powder having an oxygen content not more than 1% by weight and 1 to 20% by weight of a powdered oxide Ln2O3 (where Ln=Y, Yb or Sm) of at least one element selected from the group consisting of Y, Yb and Sm, thereby providing a powdered raw material;
molding the raw material into a molded body;
nitriding the molded body in an atmosphere containing nitrogen at a temperature of not higher than 1400xc2x0 C.; and
sintering the nitride body obtained in an atmosphere containing nitrogen at a temperature of 1700 to 1950xc2x0 C.
In this method for producing the Si3N4 sintered body, a reducing coating agent in an amount of 1 to 10% by weight based on the weight of the silicon powder is further added to and mixed with the above-described powdered raw material, and a molded body of the resultant mixture is head-treated with in a vacuum of not more than 100 Torr or in an atmosphere containing nitrogen at a temperature of 200 to 800xc2x0 C., and then the above-described nitriding and sintering can be performed. As the reducing coating agent of this case, a compound including C, H, O and metallic cations can be used, and specifically, a coupling agent including Si or Ti as the metallic cations is preferable.
Moreover, in the method for producing the Si3N4 sintered body, the content of oxygen, Y, Yb and Sm in the powdered raw material is preferably within the range of 0.1 to 2 in terms of a molar ratio of oxides, SiO2/Ln2O3, and more preferably the range is within 0.1 to 0.8.
It is thought that lattice defects in the Si3N4 crystal grains and a glass component in the grain boundary phase exert influences upon the thermal conductivity of the Si3N4 sintered body, and as the lattice defects, an existence of impurity ions such as oxygen ions which dissolve into the crystals in the state of solid solution is enumerated. Therefore, in the invention, formation of a solid solution due to the dissolution of impurity oxygen ions into the crystal grains of Si3N4 is suppressed by using the silicon powder having an oxygen content of not more than 1% by weight as a starting material so that the thermal conductivity of the Si3N4 sintered body is enhanced.
Moreover, a kind of an oxide system sintering aid which is one component of the starting material also exerts an influence upon the thermal conductivity of the Si3N4 sintered body. Namely, it has been known that since oxides of 3A group elements in the periodic table do not dissolve into the crystals of Si3N4 to form a solid solution, it is advantageous from the viewpoint of thermal conductivity. In the invention, as a result of the intensive studies on the sintering aid of the 3A group elements, it was discovered that among the 3A group elements, Y, Yb and Sm are effective for enhancing thermal conductivity and especially Yb is most effective, whereby the oxides of these elements are used as a sintering aid.
Since Y, Yb and Sm have a high strength of an ion electric field and composite oxides are formed through a strong electrostatic bonding with oxygen ions in the grain boundary phase, even at high temperature, Y, Yb and Sm act so as to prevent oxygen from dissolving into the crystal grains of Si3N4 to form a solid solution. Moreover, when these elements are added, a crystal phase which is precipitated in the grain boundary phase has a better crystallinity than one in the case where the other 3A group elements are added. By the above reason, it is thought that addition of Y, Yb and Sm provides highly enhanced thermal conductivity to the resultant Si3N4 sintered body.
The amount of addition of the oxide (Ln2O3) powder of the above-described Y, Yb and Sm (collectively expressed by Ln, hereinafter referred as the same way) is within the range of 1 to 20% by weight based on the total of the raw material powders including the silicon powder. When this amount of addition is less than 1% by weight, nitriding reaction of silicon powder is not stimulated sufficiently and the silicon particles remain after nitriding. Therefore, an elution reaction of silicon particles occurs at a higher temperature than the melting point of silicon at subsequent sintering, whereby a sintered body of 85% or more in relative density can not be obtained, so that the sintered body fails to have a thermal conductivity of 70 W/m.K or more and a three-point bending strength of 600 MPa or higher. In contrast to this, when this amount of addition exceeds 20% by weight, the amount of the grain boundary phase in the resultant sintered body obtained is increased, whereby the thermal conductivity of the sintered body is reduced.
Moreover, the ratio of oxygen and Ln in the Si3N4 sintered body is within the range of 0.1 to 0.8 in a molar ratio SiO2/Ln2O3 of SiO2 to the oxide Ln2O3 and as described in the later examples, the contents of oxygen and Ln in the sintered body accord with those in the raw material powders. In the molar ratio the amount of SiO2 is the amount when all oxygen in the sintered body is assumed to be present as SiO2. Namely, the amount of SiO2 is calculated as the product of x% by weight and a molecular ratio of SiO2/O2, when the amount of oxygen in the silicon powder is x% by weight. In the above ratio, the amount of Ln2O3 is the total amount of Y2O3, Yb2O3 and Sm2O3 powders initially added as a sintering aid. When the molar ratio of SiO2/Ln2O3 is less than 0.1, the liquid phase is not generated sufficiently at the time of sintering, whereby sintering can not be progressed, so that the strength of the resultant sintered body can not be improved sufficiently. On the other hand, when the molar ratio of SiO2/Ln2O3 exceeds 2, the amount of the liquid phase increases and the dissolution amount of Si3N4 into the liquid phase is also increased, so that the proportion of the grain boundary phase becomes large. Therefore, there is the danger that the thermal conductivity of the sintered body will be reduced.
After molding the powdered raw material prepared in such manner, the molded body is sintered at a temperature of not higher than 1400xc2x0 C. in an atmosphere containing nitrogen to nitride the silicon powder. For a gas pressure of the atmosphere containing nitrogen in this case, an approximately normal pressure (1 atm) is preferable. When the firing temperature at nitriding exceeds 1400xc2x0 C., the temperature reaches the melting point of silicon and the elution reaction of Si occurs before the nitriding reaction of silicon is finished, so that it results in that not-yet nitrided portion of silicon remains, and this silicon will lead to an reduction in thermal conductivity and strength. Therefore, the firing temperature exceeding 1400xc2x0 C. is not preferable. The lower limit of the preferable firing temperature for nitriding is 1200xc2x0 C.
The molded body (referred to as the nitrided body) nitrided in the above-described manner is fired at a temperature of 1700 to 1950xc2x0 C. in an atmosphere containing nitrogen to form a dense Si3N4 sintered body. When the firing temperature of this treatment is lower than 1700xc2x0 C., the liquid phase is not generated sufficiently and the sintering reaction does not proceed, whereby the grains do not grow and the thermal conductivity and the mechanical strength of the resultant sintered body become low. When the firing temperature exceeds 1950xc2x0 C., the growth of Si3N4 grains becomes considerable. Accordingly, although the thermal conductivity of the resultant sintered body becomes high, the mechanical strength decreases, since grown coarse grains themselves act as starting points of destruction.
Thus, the Si3N4 sintered body obtained according to the method of the invention contains a compound of at least one element selected from among Y, Yb and Sm in amounts of 0.6 to 13% by weight calculated as oxide Ln2O3 and the oxygen content in the Si3N4 crystal grains is not more than 1% by weight. Moreover, the Si3N4 sintered body has a relative density of 85 to 99.9% and a thermal conductivity of 70 W/m.K or more and a three-point bending strength of 600 MPa or more. Accordingly, the Si3N4 sintered body exhibits a high thermal conductivity coupled with a high strength.
More preferably, the Si3N4 sintered body contains xcex2-type Si3N4 crystal grains having an average grain size of 2 xcexcm or more in terms of major axis, and especially preferably all the Si3N4 crystal grains are contained as xcex2-type crystal grains in the Si3N4 sintered body. Throughout this specification, the grain or particle size of Si3N4 is expressed in major axis, unless otherwise specified. Since the xcex2-type Si3N4 has a low content of impurities dissolved in the state of solid solution therein as compared with the xcex1-type Si3N4 crystal grains and grows as xcex2-type columnar crystal grains, the thermal conductivity of the sintered body can be further enhanced.
Moreover, in the invention, a reducing coating agent is added and mixed into the raw material powders to perform a heat-treatment for reduction, thereby being able to further improve the thermal conductivity of the Si3N4 sintered body. That is, since the reducing coating agent adheres onto the surface of the silicon powder or the like to remove the oxide films on the surface of the silicon particles at the time of the reducing heat-treatment, the oxygen content in the resultant Si3N4 sintered body further is decreased, whereby the thermal conductivity can be further improved. Such a reducing coating agent may be compounds including O, H and metal cations in addition to C as a reducing component. For example, it is preferable to use a coupling agent such as silane-type or titanate-type one including Si or Ti as metal cations which are used to improve the adhesion properties at the interfaces in composite materials.
By adding the above-described reducing coating agent to the raw material powders and by heat-treating the molded body containing this agent before firing for nitriding, oxygen in the naturally-formed oxide film on the silicon particle surface is reduced by C in the reducing coating agent and is removed. However, it is not preferable to remove all oxygen of the silicon particle surface because it makes it difficult to sinter at a low temperature. Therefore, in the method of the invention, in order to allow to remain a small quantity of oxygen, the amount of addition of this reducing coating agent is controlled within the range of 1 to 10% by weight based on the weight of the silicon powder. Moreover, the heat treatment for reducing (reducing heat-treatment) is performed within the temperature range of 200 to 800xc2x0 C. in a vacuum of not more than 100 Torr or in an atmosphere containing nitrogen.
In this heat treatment for reducing, when an atmosphere containing no nitrogen is used, the heat-treatment is preferably effected in a vacuum of not more than 100 Torr. The reason is that there is the risk that when exceeding 100 Torr, oxygen in an atmosphere reacts with the reducing coating agent, so that oxygen in the silicon particle surface remain without being reduced. Moreover, the reason why the heat treatment temperature of this case is adjusted to 200 to 800xc2x0 C. is that in the case of lower than 200xc2x0 C., the reducing coating agent remains in the molded body to cover the surface of the silicon particles, whereby the subsequent nitriding reaction is prevented from progressing sufficiently. On the other hand, there is the risk that when the firing temperature exceeds 800xc2x0 C., most of all oxygen in the molded body is reduced, so that the amount of the liquid phase at sintering becomes small, whereby the densification of the sintered body will not be effected sufficiently.
As a result of this reducing heat-treatment, a part of oxygen in the silicon particle surface remains and the remaining part is reduced and removed. Moreover, the reaction is progressed in such a manner that, in subsequent nitriding, N of nitrogen gas enters into holes formed after part of oxygen has been removed. Therefore, the grain boundary phase of the prior Si3N4 sintered body are oxides such as LnpSiqOr (where p, q and r are natural numbers), whereas in the grain boundary phase of the Si3N4 sintered body prepared by the method of the invention using the reducing coating agent described above, the oxynitride expressed by LnaSibOcNd (where 2xe2x89xa6axe2x89xa64, 2xe2x89xa6bxe2x89xa63, 0xe2x89xa6cxe2x89xa67, 2xe2x89xa6dxe2x89xa64) is contained. Especially, when Ln is Yb, the thermal conductivity is greatly improved, and in this case the oxynitride expressed by Yb4Si2O7N2 is contained in the grain boundary phase.